Some combinators to express breadth-first evaluation of catamorphisms,
which allows you to stepwise evaluate the results of children. If,
during the evaluation of an alternative, a choice needs to be made between taking
the results of its children, with these combinators, you can stepwise
evaluate the children in parallel, until a choice can be made.
Until a choice is made, evaluation proceeds strictly; after a choice
is made, evaluation proceeds lazily.
What constitutes to be a step is determined by the callee.

Computation of synthesized attributes of nonterminal of type n.
It is a trace of Info-effects, that keeps track of the intermediate
states of the tree (using Pending), ending ultimately in the
synthesized values (using Final).
Operationally, we lift Info-values over Pending-values, thereby
gradually rewriting the latter, until it results in a Final.